Semiconductor devices are used in many electronic and other applications. Semiconductor devices comprise integrated circuits that are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, and patterning the thin films of material to form the integrated circuits.
One type of semiconductor device is a memory device, in which data is typically stored as a logical “1” or “0.” A memory device may be static or dynamic. A dynamic memory device needs to be refreshed to “remember” the data, whereas a static memory device does not need to be refreshed to retain stored data.
One type of static memory device, also referred to in the art as a non-volatile memory (NVM) device, is a floating gate memory device. A floating gate memory device can be either erasable programmable read-only memory (EPROM) or electrically erasable programmable read-only memory (EEPROM). Both these type of floating gate memories rely on a charge stored in the floating gate (or a charge trap layer) by suitable application of a bias to the various terminals of the device. The charge may be stored by a number of mechanisms such as carrier tunneling and/or injection. The charge may be removed either electrically as in EEPROM devices or by an external source such as an ultra violet light. The presence of this charge in the floating gate determines the state of the memory as logical “1” or “0.” Flash EEPROM memories are called such due to their fast program and erase times (as in a lightning flash).
The floating gate devices are usually stacked in large arrays to form memory cells such as flash memory cells. Based on the stacking or layout of the floating gate transistors, flash memory may comprise a NOR, NAND, or an AND memory architecture. As an example, most commercial memory cards such as memory sticks comprise NAND flash memory cells. Flash memory is amongst the most popular memories available in the market today. The popularity of flash memory arises partly due to its compatibility with existing CMOS process flows. Flash memory is simply a field effect transistor, except it has a poly-silicon floating gate (or Silicon Nitride charge trap layer) sandwiched between a tunnel oxide and an inter-poly oxide to form a charge storage layer. However, flash memory devices have some disadvantages or bottlenecks that need to be overcome for continued success.